An abnormally high load may be placed on a marine engine when a marine vessel is travelling forward at a high rate of speed and is rapidly shifted into reverse gear. This is because the momentum of the vessel remains in the forward direction for a brief period of time. This forward momentum typically urges the vessel's propeller to continue spinning in the direction that it was spinning prior to the shift into reverse gear.
When the engine is in reverse gear, however, the engine attempts to drive the propeller in the opposite direction. The momentum of the vessel thus places a load on the propeller, which is transferred back to the engine via a drive shaft. The load is generally proportional to the speed of the vessel and the quickness of the shift from forward to reverse gear. Generally, lowering the speed of the vessel or slowing the duration of the shift allows the vessel to be travelling more slowly when the shift into reverse occurs, with a proportionally lower load being placed on the propeller.
Further exacerbating the situation, in reverse gear the range of engine speeds available may be relatively low, meaning that a relatively small amount of power is provided by the engine to overcome any loading. In some situations, when this small amount of power tries to overcome the load on the engine, the engine speed may drop below a minimum desired engine speed, causing, for example, lugging of the engine, which is generally undesirable.